1. Field of the Invention
The present invention relates to a chemical sensor based on metal nanoparticles encapsulated by at least two kinds of mixed ligands and a sensor array using the same.
2. Discussion of Related Art
Depending on the state of the analytes sensing technology can be classified into ‘electronic nose’ and ‘electronic tongue’ corresponding respectively to gas phase and liquid phase. Compared with the conventional single chemical sensor whose sensing capability is limited to an analyte composed of a single molecule, a sensor array composed of a number of sensors enables pattern type of sensing for an analyte composed of various kinds of molecules, thus performing mammalian level of sensing. [Julian W. Gardner and Philip N. Bartlett, Electronic Noses: Principles and Applications, Oxford University Press: Oxford, U.K., (1999)].
The sensor array for the pattern type of sensing needs various sensors. Metal oxide sensors have been typically for used that purpose in which their oxidation and reduction characteristics are modified by varying metal catalysts added. [Keith J. Albert, Nathan S. Lewis, Caroline L. Schauer, Gregory A. Sotzing, Shannon E. Stitzel, Thomas P. Vaid, and David R. Walt “Cross-Reactive Chemical Sensor Arrays” Chem. Rev. 100 (2000) 2595-2626]. However, the limit in diversifying the chemical selectivity and the need for power required for high temperature in operation are understood to be the major difficulties in miniaturizing the array systems.
Recently, various sensors or sensor technologies have been developed in the senor array field to overcome those limitations. Among them carbon black-polymer composites and metal nanoparticles encapsulated by molecular monolayer have been gaining interests as sensor materials to overcome the above-mentioned limitations. For example, in the carbon black-polymer complex sensor, conductive carbon black particles are dispersed in a non-conductive polymer matrix to form a sensor film. When the analyte molecules are in contact with the sensor films, the polymer matrix is swollen to increase the distance between the conductive carbon black particles causing increase in the electrical resistance of the sensor composites. [Mark C. Lonergan, Erik J. Severin, Brett J. Doleman, Sara A. Beaber, Robert H. Grubbs, and Nathan S. Lewis “Array-Based Vapor Sensing Using Chemically Sensitive, Carbon Black-Polymer Resistors” Chem. Mater. 8 (1996) 2298-2312]. The easy manipulation of chemical selectivity provided by the diversity of the polymers and the various methods in providing films using for examples spin-coating and dip-coating the composite solutions, are understood to be the major advantages as materials for array technology.
An attempt to use the sensor array in the disease diagnosis is recently drawing attention. The disease diagnosis using this sensor array has been acknowledged as a key merit in that a non-invasive real-time disease diagnosis may be made possible simply using human breath or secretions [Maximilian Fleischer, Elfriede Simon, Eva Rumpel, Heiko Ulmer, Mika Harbeck, Michael Wandel, Christopher Fietzek, Udo Weimar and Hans Meixner, “Detection of volatile compounds corrected to human disease through breath analysis with chemical sensors” Sensors and Actuators B 83 (2002) 245-249]. However, in this sensor array application field, currently, the most difficulty is a limitation of the sensor sensitivity. For example, the gas concentrations related to diseases are on the order of a few ppm to a few ppb. Since the carbon black-polymer complex has a sensitivity limitation in the order of hundreds of ppm over most of the analytes, overcoming the sensitivity limitation is required for use in the disease diagnosis.
With respect to this sensor limitation, a monolayer metal nanoparticle sensor is advantageous. Recently, a gold nanoparticle (2 nm in diameter) sensor encapsulated with octanethiol has been reported to successfully detect a several ppm of toluene [U.S. Pat. No. 6,221,673 to Hank Wohltjen and Arthur W. Snow; Hank Wohltjen and Arthur W. Snow, “Colloidal Metal-Insulator-Metal Ensemble Chimiresistor Sensor” Anal. Chem. 70 (1998) 2856-2859]. This octanethiol-gold nanoparticle, however, has been reported to have a good sensitivity for a non-polar molecule such as toluene or CCl4, while it has a poor sensitivity toward polar molecules. To improve the sensitivity for the polar molecule, the nanoparticles composed of alcohol (—OH) or ethylene oxide ligands has been reported [H-L Zhang, S D Evans, J R Henderson, R E Miles and T-H Shen “Vapour sensing using surface functionalized gold nanoparticles” Nanotechnology 13 (2002) 439-444; Edward E. Foos, Arthur W. Snow, Mark E. Twigg, and Mario G. Ancona “Thiol Terminated Di-, Tri-, and Tetraethylene Oxide Functionalized Gold Nanoparticles: A Water-Soluble, Charge-Neutral Cluster” Chem. Mater. 14 (2002) 2401-2408]. Although the sensitivity was improved (20 ppm for ethanol), problems with regard to response time and signal stability still remain.